In excerpts from "A Higher Loyalty: Truth, Lies, and Leadership" published by Axios, Comey writes that he wondered if there was something more "complicated" to Trump's relationship with Putin and Russia.

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"I had often wondered why, when given numerous opportunities to condemn the Russian government’s invasions of its neighbors and repression — even murder — of its own citizens, Trump refused to just state the plain facts," Comey writes.

"Maybe it was a contrarian streak or maybe it was something more complicated that explained his constant equivocation and apologies for Vladimir Putin," he added.

Democrats and other critics have pointed to Trump's statements and actions on Russia as proof of alleged collusion between the Trump campaign and Russia during the 2016 presidential election.

Last month, the president was heavily criticized by both parties after he congratulated Putin on winning reelection, despite numerous concerns over the fairness and openness of Russia's elections.

Trump has defended his soft rhetoric on Putin, calling it "a good thing" that America can work with Russia against threats such as the Islamic State in Iraq and Syria.

"I called President Putin of Russia to congratulate him on his election victory (in past, Obama called him also). The Fake News Media is crazed because they wanted me to excoriate him. They are wrong! Getting along with Russia (and others) is a good thing, not a bad thing," Trump tweeted in March.